"We don't have any projected rainfall totals," says Roger Gass, a meteorologist with the National Weather Service Bay Area. "The reality is there are still a lot of unknowns. There are some models indicating we might not get any rain at all if the system moves south. It remains difficult to say anything with certainty. It is early in the season and it's typical at this time of year for our models to not all be aligned."

The shift in weather comes as a blocking pattern acting as a dam in the jet stream breaks down beginning Friday, allowing for atmospheric flow and new systems to move into the region.

"Late Friday into Saturday, we're going to get more progressive features moving over the area," explains Drew Peterson, a meteorologist with the National Weather Service Bay Area. "Typically that brings in stormier weather."

In recent days, inland areas of the Bay Area have seen seasonably warm weather with his in the 80s and 90s. Those inland areas are going to be up to 20 degrees cooler starting Friday.